Timing based servo system for magnetic tape systems

ABSTRACT

A track following servo system is disclosed for use with magnetic tape systems in which magnetic servo track patterns contain transitions recorded at more than on azimuthal orientation across the width of the servo track. The timing of a signal derived from reading at any point across the servo track. The pattern is read by a servo read head whose width is small compared to the servo track pattern. The combination of a wide servo pattern and a narrow servo read head offers excellent position sensing linearity and dynamic range. In the preferred embodiment, the servo read head is also narrow with respect to the data tracks, which provides the additional advantages of superior immunity to position sensing errors caused by defects or temporal variations in the servo read head, defects in the servo pattern on the tape, wear of the head or tape, or debris collection on the head or tape. Position sensing with this system is achieved by deriving a ratio of two servo pattern intervals and therefore is insensitive to tape speed during reading. The servo patterns may include spacing intervals recognizable for error detection and correction purposes. Servo tracks are recorded using a patterned multiple gap servo write head whose magnetic gaps have geometries appropriate to generate the desired servo patterns. The patterned gaps of the servo write head are produced by photolithographically defined electroplating of permalloy on a ferrite ring head structure.

This application is a division of application Ser. No. 08/270,207, filedJun. 30, 1994, now U.S. Pat. No. 5,689,384.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to recording and reading data frommagnetic storage media and, more particularly, to servo control systemsthat maintain the position of a magnetic head relative to tracks inmagnetic storage media.

2. Description of the Related Art

The recording and reading of data in tracks on magnetic storage mediarequires precise positioning of magnetic read/write heads. Theread/write heads must be quickly moved to, and maintained centered over,particular data tracks as recording and reading of data takes place. Themagnetic heads can record and read data as relative movement occursbetween the heads and the magnetic storage media in a transducingdirection. The heads are moved from track to track across the width ofthe tracks in a translating direction, which is perpendicular to thetransducing direction.

For example, a recordable disk typically contains concentric data tracksand is rotated beneath a magnetic head. The direction of rotationdefines the transducing direction. Radial movement from track to trackdefines the translating direction. A magnetic tape typically containsdata tracks that extend along the length of the tape, parallel to thetape edges, in the transducing direction. In magnetic tape helical scansystems, however, the tape is moved beneath heads that are moved at anangle across the width of the tape, the diagonal direction defining thetransducing direction.

Storage devices that read and record data on magnetic media typicallyuse servo control systems to properly position the data heads in thetransducing direction. The servo control systems derive a positionsignal from a servo magnetic head that reads servo control informationrecorded in servo tracks on the storage media. Typically, the servocontrol information comprises two parallel but dissimilar patterns. Theservo head follows the boundary between the two dissimilar servopatterns, which are recorded in alignment with the data tracks. When theservo head is centered relative to the boundary between the servopatterns, the associated read/write head is centered relative to thedata track.

The servo patterns might comprise bursts of half-width magnetic fluxtransitions, extending halfway across the servo track, that havedifferent phases or frequencies. These patterns are often referred to as"half-tracks" because a single servo position is defined by an adjacentpair of the patterns. Generally, the servo head has a width greater thanor equal to approximately one-half servo track. With a half-width servohead it is readily possible to determine which direction to move thehead for centering up until the head has moved more than one-half trackoff center. Servo heads that are less than one-half track width wouldnot be able to determine which direction to move as soon as the head wascompletely over one half of the servo track or the other. Servo headsthat are greater than one-half track width are most commonly used withimbedded servo systems, which use the same read head for servo and fordata. With such systems, every other patterns made different to avoidthe problems of the head running into an adjacent track pattern, whichthen would not be able to determine which direction in which to move.

An alternative to the half-track servo control approach is described inU.S. Pat. No. 3,686,649, to Behr, which describes a disk drive servocontrol system that uses servo control information comprising lines ofmagnetic flux transition that extend across a servo track width at twodifferent angles from a line parallel to a disk radius. A pair of suchtransition lines define a control zone in the form of a symmetrictrapezoid. A control head detects a positive-transition. The signal thusgenerated comprises a pulsed position signal that can be compared with areference signal to indicate how far the control head has deviated fromthe servo track centerline. The system is said to permit more than 200tracks per inch on a storage disk. Nevertheless, there is a demand fordisk storage devices and tape storage devices of greater and greaterstorage density. For example, conventional disk drives can provide 5000tracks per inch.

The half-track servo control approach has been found to be generallysatisfactory for direct access storage devices, such as disk drives.Tape storage systems operate under unique characteristics that increasethe difficulty of providing higher storage densities. In magnetic tapestorage systems, the storage media/magnetic head interface is not asclean as the environment typically found in disk systems and, unlikemost disk systems, the magnetic tape runs substantially in contact withthe magnetic head. The relatively dirty environment and continuouscontact between the media and the head, as well as the relatively largewidth of the servo head, produces significant wear and scratching ofboth the media and the servo head and produces localized build-up ofcontaminants on the surfaces of both. As a result, the spatial responseof the servo head to the servo control information changes with time,both gradually as a result of wear over time and suddenly as a result ofinteraction with contaminant debris.

Changes in the servo head spatial response cause errors in the positionsignal, so that a position signal can indicate to track misregistrationwhen the servo head actually is displaced from the servo trackcenterline. Errors in the position signal are typically difficult todetect from the position signal itself. As a result, redundant servotracks are often used for increased reliability, wherein the servocontrol system uses the position signal data only if the data from twoor more redundant tracks agree. Redundant servo tracks reduce the tapestorage media surface available for data recording and requires moreheads and supporting electronics.

From the discussion above, it should be apparent that there is a needfor a servo control system that is especially suited to the magnetictape environment, that reduces the magnitude of position signal errordue to wear on the servo head and debris, and that permits positionsignal errors to be detected more easily. The present inventionsatisfies this need.

SUMMARY OF THE INVENTION

In accordance the present invention, a track-following servo controlsystem in a magnetic media storage device derives head positioninformation from one or more specially patterned servo tracks. The servopatterns are comprised of magnetic transitions recorded at more than oneazimuthal orientation in a servo track, such that the timing of theservo position signal pulses derived from reading the servo pattern atany point on the pattern varies continuously as the head is moved acrossthe width of the servo track. The timing of pulses generated by theservo read head is decoded by appropriate circuitry to provide a speedinvariant position signal used by the servo system to position the dataheads over the desired data tracks on the storage media.

In one aspect of the invention, the servo pattern is comprised of arepeating cyclic sequence containing two different transition azimuthalorientations. For example, the pattern may comprise straight transitionsessentially perpendicular to the length of the track alternating withazimuthally inclined or sloped transitions. That is, the azimuthallysloped transitions extend across the width of a track at an angle to thehead transducing direction. The relative timing of transitions read by aservo read head varies linearly depending on the head position withrespect to the center of the track. Speed invariance is provided bydetermining the ratio of two timing intervals. In particular, the ratiocan be determined by normalizing the variable time interval betweendissimilar transitions with the internal measured between liketransitions. Maximum dynamic range and linearity are obtained by using aread head that is narrow with respect to the width of the servo trackpattern and the data track width. Synchronization of the decoder to theservo pattern is accomplished by providing periodic gaps called spacingintervals or synchronization gaps in the pattern that are recognized aspattern starting points.

In another aspect of the invention, error detection and correction areaccomplished through recognition of servo pattern sequences. Forexample, if the servo pattern contains a predetermined number oftransitions between synchronization gaps, a failure to encounter theexpected number of transitions between gaps indicates faulty servo trackreading. Similarly, the timing of various intervals within a servopattern sequence must match a known format: failure to match withincertain parameters indicates erroneous servo track reading. Upondetection of errors, the system may correct the false information bysubstituting information from a different (redundant) servo track,temporarily substituting information of an estimated value, or by othermeans.

Servo patterns on a tape storage media can be generated using a multiplegap servo write head. The gaps of the head contain geometriesappropriate to generate the servo pattern features described above. Forservo patterns comprised of straight transitions at two differentazimuthal orientations, for example, a dual gap head having one narrowstraight gap at each orientation is sufficient. In one feature of theinvention, the patterned gaps of the head are produced byphotolithographically defined plating of permalloy material on a ferritering head structure. Pulses of current through the windings of the writehead transfer the geometric pattern of the gaps on the head to identicalmagnetization patterns on the tape. Appropriate timing of the pulsesgenerates the desired pattern sequences.

In accordance with the invention, using a servo read head that is narrowcompared to the data tracks written on the storage media minimizestracking errors due to false position signals. Defects in the and wearof the servo read head or servo patterns on the storage media causeminimal position errors if the servo read head is narrow. Likewise,temporary or permanent collection of debris on either the storage mediaor servo read head cause minimal position sensing errors if the servoread head is narrow relative to the data track widths. The system isespecially suited for use with dedicated servo tracks that are alwaysseparate from data tracks. However, the system may be used in embeddedservo systems as well.

In another aspect of the invention, the servo control system detectsmagnetic flux transitions having a first transition polarity and ignoresmagnetic flux transitions having a second transition polarity. Thus, theservo control information pattern groups are timed only betweentransitions that have the same polarity. This avoids shifts in thetiming of opposite polarities of transitions that can occur due toasymmetries in the fabrication of the servo write head, in the actualservo control information writing process, the nature of the magnetictape, and the read heads themselves. If desired, signal-to-noise ratioscan be further improved by using the redundant second set of oppositepolarity transitions.

Other features and advantages of the present invention should beapparent from the following description of the preferred embodiments,which illustrate, by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a tape drive storage device andassociated tape cartridge constructed in accordance with the presentinvention.

FIG. 2 is a schematic representation of the magnetic head and servocontrol system of the tape drive and cartridge combination illustratedin FIG. 1.

FIG. 3 is a schematic representation of an alternative head assemblyarrangement in accordance with the invention.

FIGS. 4, 5, and 6 are representations of three alternative servopatterns constructed in accordance with the present invention.

FIG. 7 is a graph of the servo information signal generated by themagnetic head illustrated in FIG. 2.

FIG. 8 is a representation of the servo head as it tracks the servopattern illustrated in FIG. 6 and a graph of the head output signal itgenerates.

FIG. 9 is a representation of the servo head as it tracks an illustratedfourth alternative servo pattern constructed in accordance with thepresent invention and a graph of the head output signal it generates.

FIGS. 10, 11, and 12 are block diagrams of a position signal decoder forthe servo control system illustrated in FIG. 2.

FIGS. 13, 14, 15, and 16 are block diagrams of an alternate positionsignal decoder for the servo control system illustrated in FIG. 2.

FIG. 17 is a representation of the servo head as it tracks the servopattern illustrated in FIG. 9, along with a representation of the headoutput signal it generates and the corresponding A and B signalintervals.

FIG. 18 is a chart of data stored in the system illustrated in FIG. 13for use with demodulating the servo pattern illustrated in FIG. 17.

FIG. 19 is a representation of a drum system for recording the servopatterns onto magnetic storage tape.

FIG. 20 is a representation of the magnetic flux transitions that can berecorded onto a portion of magnetic tape by the system illustrated inFIG. 19.

FIG. 21 is a representation of a multi-gap head that can record theservo pattern illustrated in FIG. 9.

FIG. 22 is a cross-section of the head illustrated in FIG. 21.

FIG. 23 is a plan view of the servo pattern gap region of the headillustrated in FIGS. 21 and 22.

FIG. 24 is a schematic representation of a servo write head writing amagnetic tape constructed in accordance with the invention.

FIG. 25 is a schematic representation of a recording system forproducing a magnetic tape in accordance with the present invention.

FIG. 26 is a schematic diagram of the recording system illustrated inFIG. 25.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a timing based servo tape system 10 constructed inaccordance with the present invention. The system includes a tape drive12 that accepts a tape data cartridge 14 and is connected to a hostprocessor 16 by a data cable 18. The tape cartridge comprises a housing19 containing a loop of magnetic tape 20. The system is constructed foruse with servo control information comprising a repeating servo patternof magnetic flux transitions that are recorded in tracks on the magnetictape 20 in the data cartridge and extend across the width of the trackssuch that a servo position information signal generated by reading theservo control information varies continuously as a magnetic servo trackread head is moved across the width of a track in a translatingdirection, thereby indicating the relative position of the head withinthe track. The tape drive 12 can read the servo control information andgenerate a position signal to control the position of an associated dataread head, or can write the servo control information into tracks on adata cartridge using a magnetic servo write head, or can do both. Thesystem is optimized for the magnetic tape environment, so that themagnitude of the position signal error due to wear and debris is reducedand such errors are easier to detect.

The tape drive 12 includes a receiving slot 22 into which the cartridge14 is inserted. The host processor 16 can comprise, for example, apersonal computer such as the IBM Corporation "PS/2" personal computer,or can be a workstation such as the IBM Corporation "RS6000"workstation, or can be a mini computer, such as the IBM Corporation"AS400" computer. The tape drive 12 preferably is compatible with suchhost processors and, for example, tape library systems that employ tapecartridges, such as IBM Corporation "3480" and "3490" tape drive units.The tape cartridge 14 can assume any one of a variety of cartridgeformats, including, for example, conventional 8 mm, 4 mm, 1/4-inch, and1/2-inch data cartridge formats.

FIG. 2 is a view looking down on a portion of the magnetic tape 20 ofthe cartridge 14 (FIG. 1) past a head assembly 24 of the tape drive unit12. The tape is shown in phantom with dashed lines where it passesbeneath the head assembly. The head assembly is shown in solid lines andincludes a relatively narrow servo head 26 that detects a servo patternrecorded in a servo track 27 of the tape. Also shown, for purposes ofillustrating relative size, is a data read head 28 of the head assemblythat is positioned over a data track region 29 of the tape containingmultiple data tracks for reading data recorded in a data track. FIG. 2shows a single servo read head and a single data read head forsimplicity of illustration. Those skilled in the art will appreciatethat most tape systems have multiple servo tracks, multiple servo readheads, and multiple data read and write heads.

In FIG. 2, the servo track centerline 30 is indicated, extending alongthe length of the tape 20. FIG. 2 shows that the servo read head isrelatively narrow and has a width substantially less than the width ofthe servo track 27. In particular, in the preferred embodiment the servoread head has a width that is less than one-half the width of a singledata track (not illustrated), which typically is more narrow than aservo track.

In FIG. 2, the transducing direction of tape-head relative movement, inwhich the servo read head 26 can read the servo pattern, occurs when thetape 20 is moved linearly with respect to the head, along the length ofthe track 30. When such movement occurs, the servo pattern of magneticflux transitions is detected by the servo read head so that is generatesan analog servo read head signal that is provided via a servo signalline 34 to a signal decoder 36. The signal decoder processes the servoread head signal and generates a position signal that is delivered via aposition signal line 38 to a servo controller 40. The servo controllergenerates a servo mechanism control signal and provides it via a controlline 42 to the head assembly 24. A servo mechanism of the head assemblyresponds to the control signal from the servo controller by moving thehead 26 laterally across the width of the servo track 30 in thetranslating direction. The servo controller 40 monitors the positionsignal from the signal decoder 36 and generates the control signalnecessary to reach the derived position, so that the control signalsignals the signal when the head is at the desired target.

FIG. 3 shows a multiple servo track, multiple head system constructed inaccordance with the present invention. The system is similar to thatshown in FIG. 2 with the following exceptions. The FIG. 3 head assembly24' includes a data read head 28a and a data write head 28b for readingand writing data, respectively, in a data track of the tape data region29. The tape 20' illustrated in FIG. 3 includes a second servo track 27'in addition to the first servo track 27, the servo tracks being placedon opposite sides of the data region 29. The centerline 30' of thesecond servo track also is shown. FIG. 3 shows that the head assembly24' also includes a second servo read head 26' for reading servoinformation recorded in the second servo track 27'. It should be notedthat the head assembly 24' produces two servo signals, one for eachservo read head. The head assembly provides the servo signal from thefirst servo read head 26 over a signal line 34 to a correspondingdecoder 36 and provides the servo signal from the second servo read head26' over a second signal line 34' to a corresponding decoder 36'. Theserespective decoders provide their position signals to the servocontroller 40. It should be noted that most tape systems include amultiplicity of data read and write heads and that only a single pairare shown in FIG. 3 for purpose of illustration.

As noted above, servo patterns in accordance with the present inventioncomprise magnetic flux transitions that extend across the width of theservo track such that the servo read head signal produced by reading thepattern varies continuously as the servo read head is moved across thewidth of each servo track. FIG. 4, 5, and 6 show alternative embodimentsof servo patterns in accordance with the present invention. Thoseskilled in the art will recognize that the dark vertical bands,hereafter called stripes, represent magnetized areas of recordedmagnetic flux that extend across the width of a servo track and that theedges of the stripes comprise flux transitions that are detected togenerate the servo read head signal. The transition have two magneticpolarities, one on each edge of a stripe. When the servo read headcrosses a transition, it produces a pulse whose polarity is determinedby the polarity of the transition. For example, the servo head mightproduce positive pulses on the leading edge of each stripe (onencountering a stripe) and negative pulses on the trailing edge (onleaving a stripe). Each servo pattern comprises a repeating sequence ofdifferent stripes having at least two orientations across the width ofthe track such that the first orientation is not parallel to the secondorientation.

For example, in FIG. 4, the servo pattern 44 comprises an alternatingsequence of first stripes 46 that extend across the width of the tracksubstantially perpendicular to the transducing direction of a track andsecond stripes 48 that have an azimuthal slope with respect to the readhead. That is, the second stripes are at a slope relative to thelengthwise track centerline 49. The pattern 50 illustrated in FIG. 5comprises an alternating sequence of straight first stripes 52 that areoriented perpendicular to the track centerline and chevron-shaped secondstripes 54 having two legs each with an azimuthal slope symmetricallyfrom the other about the track centerline 55. That is, the pattern 50comprises a band that can be characterized as being formed from twotracks that are reflections of each other, each track including one leg54a or the other 54b of the chevrons. The FIG. 6 pattern 56 compriseschevron-shaped first 58 and second 60 stripes that are placedback-to-back so as to form a diamond-shaped pattern that is symmetricabout the track centerline 62. It should be apparent that this pattern56 also can be characterized as a servo band comprised of two servotracks reflected about a band centerline.

With each one of the servo patterns 44, 50, 56 illustrated in FIGS 4-6,a magnetic servo read head that is positioned above the tape 20 as thetape is moved linearity with respect to the head in the transducingdirection generates an analog servo read head signal having peaks whosepeak-to-peak timing varies as the head is moved across the width of thetrack in the translating direction. As described more fully below, thevariation in timing is used to determine the relative position of themagnetic servo read head within the servo track.

The servo patterns 44, 50, 56 illustrated in FIGS. 4-6 include first andsecond stripes that define first and second intervals, referred to as Aintervals and B intervals, respectively, that are used to generate aposition signal that is independent of tape speed. The position signalis generated by timing the intervals and calculating their ratio. Forthese patterns, an A interval is defined as the interval along the tapetransducing direction from a stripe of one type to the next stripe ofthe other type, while a B interval is defined as the interval along thetape transducing direction between two stripes of the same type. Itshould be clear that the timing intervals from stripe to stripe willvary as the servo read head is moved in the translating direction,across the width of the track. It also should be noted that only the Aintervals vary; the B intervals are constant, regardless of position.

Thus, in FIG. 4, the first A interval, which will be referred to as A1,extends from the first perpendicular stripe to the first stripe havingan azimuthal slope and the first B interval B1 extends from the firstperpendicular stripe to the next perpendicular stripe. Subsequent servopattern intervals A2, A3, . . . and B2, B3, . . . can be similarlydefined. In FIG. 5, the first A interval A1 extends from the firstperpendicular stripe to the first chevron-shaped stripe while the firstB interval B1 extends from the first perpendicular stripe to the secondperpendicular stripe. The second A interval, A2, extends from the secondperpendicular stripe to the second chevron-shaped stripe. The second Binterval, B2, extends from the second perpendicular stripe to the thirdperpendicular stripe. In FIG. 6, the first A, interval A1 extends fromthe first chevron, comprising the left side of a first diamond, to thenext chevron, comprising the right side of the first diamond, while thefirst B interval B1 extends from the left side of the first diamond tothe left side of the second diamond. The second A interval A2 extendsfrom the left side of the second diamond to the right side of the seconddiamond. The second B interval B2 extends from the left side of thesecond diamond to the left side of the third diamond. It should be notedthat the last stripe is not used to define an interval.

A servo control system constructed in accordance with the presentinvention provides a means of determining the position of the servo readhead relative to the beginning and ending of the servo pattern.Determination of the position within the pattern permits the system toknow the nature of the next stripe that will be read and to performerror detection and, if desired, error correction. In terms of the firstservo pattern 44 illustrated in FIG. 4, for example, the system willknow whether the next stripe to be read is a straight transition or isan azimuthally sloped transition. In the preferred embodiment, theposition determination is provided by a periodic synchronization featurein the servo pattern that is detected by the servo decoder.

In the patterns illustrated in FIGS, 4, 5, and 6, the synchronizationfeature comprises spacing intervals between groups of stripes. Thespacing intervals are transition free, so that no stripes occur in thetransducing direction for an interval greater than the maximum intervalbetween any two stripes within a group. If desired, information otherthan servo control pattern data can be placed in the spacing intervals.For example, if the gaps between strips have at least two differentlengths, information may be written in the synchronization featurespacing intervals as a serial code of gap lengths. Such informationmight be used to indicate data block locations, tape longitudinalposition, or other information useful to the operation of the drive. Theservo control system can be synchronized with the spacing intervals toprovide position determination because the system will know that thenext magnetic flux transition after a spacing interval is aperpendicular group stripe. The groups of stripes between successivespacing intervals are referred to as "servo bursts". Each servo burstcontains a predetermined number of stripes and transitions, which can beused in error detection and correction, as described further below. Thenumber of stripes per burst provides adequate servo controlsynchronization while efficiently using the tape media, such that asynchronization feature is not needed after every pair of differentstripes for proper synchronization.

For example, in FIG. 4, a first servo pattern group 66 and a secondservo pattern group 68 are illustrated. The first and second servopattern groups are separated by a synchronization feature comprising aspacing interval 70. The spacing interval extends along the tape in thetransducing direction for an interval greater than an A interval, whichis the interval from a stripe of the first perpendicular orientation toa stripe of the second azimuthally sloped orientation. Similarly, FIG. 5shows a spacing interval 72 between the servo bursts 7, 76 and FIG. 6shows a start gap 78 between servo bursts 80, 82. As noted above,information other than servo control pattern data is written in theseintervals.

To reduce the chance of head irregularities and control system anomaliesfrom distorting the servo read signal, the servo control system inaccordance with the present invention times the A and B intervals onlybetween magnetic flux transitions having the same polarity. This is donebecause, for example, asymmetries in the fabrication of the servo writehead, variations in the actual servo writing process, and otherdifficulties due to the nature of the tape itself or of the read headscan cause apparent shifts in the timing of transitions having oppositepolarities. Timing only between transitions of like polarity eliminatestiming errors due to differences between the polarities. For example,only transition pulses such as generated by the read head in movingacross the leading edge of a stripe might be used. Transition pulsesgenerated by moving across the trailing edge of a stripe are ignored.

The signal-to-noise ratio can be further improved by using the redundantset of second transitions of opposite polarity. In such a case, aredundant servo pattern decoding system would be provided to decode theposition signal separately from the magnetic flux transitions of bothpolarities. For purposes of this detailed description, the decodingsystem associated with one polarity will be described. It should beunderstood, however, that a similar decoding system could be providedfor the transitions having opposite polarity.

FIG. 7 shows a graph of the analog servo read head signal 84 generatedby the magnetic head illustrated in FIG. 2 as it reads the servo patternillustrated in FIG. 4. FIG. 7 shows that a first servo read head signalpeak 86 occurs as the servo read head crosses the leading edge of thefirst stripe of FIG. 4. A first negative peak 88 in the servo read headsignal occurs as the servo read head crosses the trailing edge of thefirst stripe in FIG. 4. This second transition polarity is ignored. Theremaining description of the servo control system will relate todetecting only the positive peaks of the servo read head signal.

FIG. 8 illustrates the diamond pattern of FIG. 6 showing a path 90followed by the servo read head and, below it, the corresponding servoread head signal 92 generated by the magnetic servo read head as itcrosses the servo pattern stripes, with the A and B intervals indicated.As described above, each successive A interval is referred to as A1, A2,and so on and the B intervals are similarly referred to as B1, B2, andso on. FIG. 8 illustrates that a positive peak is generated for eachstripe crossed and defines the pattern intervals, whereas the downwardpeaks are ignored in determining the timing intervals for generation ofthe position signal. FIG. 8 indicates that the servo pattern isapproximately 408 microns across and 434 microns long.

FIG. 9 shows an alternative nested, or interleaved, diamond pattern 94,with a representation of the path 96 followed by the servo read head,beneath which is a representation of the head output signal 97 generatedas the head crosses the servo pattern bands, illustrating the A and Bintervals. The interleaved diamond pattern comprises a sequence of fiveinterleaved diamonds, formed by a band of chevron-shaped transitions,which is followed by four interleaved diamonds. This sequence isrepeated to form the servo pattern.

The groups of five diamonds and four diamonds illustrated in FIG. 9 areseparated by relatively short spacing intervals 99 that are wider attheir narrowest point than the maximum separation between any two likestripes within an interleaved group and between any two diamond groups.It also should be apparent that another type of pattern gap having notransitions is located in the FIG. 9 pattern internal to a group ofdiamonds. These internal gaps 98 can be readily distinguished by controlcircuitry of the decoder because they occur between a sequence of fourdiamonds and five diamonds, or between a sequence of five diamonds andfour diamonds. In contrast, the spacing intervals 99 can be recognizedbecause they occur only after two sequences of stripes having an equalnumber of stripes, such as after two 4-stripe groups or after two5-stripe groups.

The servo pattern of the preferred embodiment is given by FIG. 9. Thedimensions are as follows: The stripe width in the transducing directionis 2.5 μm. The period of stripes within a group is 5 μm. The width ofthe servo pattern perpendicular to the transducing direction is 408 μm,divided into two symmetric halves of width 204 μm. The stripes areinclined at an angle of 7.4° relative to a line perpendicular to thetransducing direction. In the following dimensions, all lengths aremeasured from the leading edge of a stripe to a leading edge of anotherstripe: the spacing interval 99 between diamonds is 10 μm at the closestapproach: the internal gap 98 in a four-diamond group is 15 μm; theinternal gap 98' in a five-diamond group is 10 μm.

FIG. 9 illustrates that an A interval is defined to extend from a stripeon the left side of a diamond to a corresponding stripe on the rightside of the diamond. For example, the first A interval A1 extends fromthe first stripe of the left side of the first diamond to the firststripe of the right side of the first diamond. The corresponding Binterval extends from a stripe on the left side of a diamond to thecorresponding stripe on the left side of the next diamond.

The pattern 94 illustrated in FIG. 9 makes maximum use of the tracklength to generate a position signal. The pattern repeats every 221microns, thus, the sampling period is only 221 microns long, as comparedwith the longer sampling periods of the other illustrated servopatterns. Because each interleaved diamond of the FIG. 9 servo patterncontains a predetermined number of stripes, the synchronization featurespacing interval can be detected by counting the number of stripespassed by the servo read head. Grouping the pattern into groups of fourdiamonds followed by five diamonds permits the decoder to determine thelocation of the head relative to the track in the transducing direction.More particularly, the decoder can synchronize itself, even if it missesa stripe, because it can expect that, after it receives two bursts offive strips, it next will receive two bursts of four stripes, then twomore bursts of five stripes, and so forth. This advantageously permits arelatively simple error detection and correction scheme to beimplemented.

The dimensions of the pattern 94 illustrated in FIG. 9 represent apreferred design which balances three servo requirements: servo patternwidth, sample rate, and position signal noise. The pattern width(indicated in FIG. 9 as 408 microns) determines the range of the servoread head signal. This range can be the width of several data tracks(not illustrated). In this embodiment the servo pattern width is equalto approximately eight data track widths so that one servo read elementcan be used to position a given data read head element over eightdifferent data tracks.

The sample rate of the servo read head signal is determined by thelength of the servo pattern and the tape speed. In the preferredembodiment, the servo pattern is 221 microns long. It yields two datapoints in that space, one at the end of the interval B4 and one at theend of the interval B8. At a typical tape speed of approximately 2.0m/s, this yields a sample rate of 18,100 samples per second. The samplerate requirement is determined by the rest of the components of thetrack-following servo loop. If the sample rate is too low, the dynamicresponse of the loop must be relaxed in order to maintain enough phasemargin in the system for adequate control loop stability.

The position signal noise is determined by three factors: the noise inthe measurement of the transition interval times, the number oftransition interval times measured per sample, and the scaling factorwhich converts transition interval time to position signal. The noise inthe measurement of the transition interval times is governed by suchfactors as media noise and electronics noise and is largely independentof the pattern dimensions. The noise is considered a constant in thisdiscussion. The number of transitions measured affects the positionsignal noise because of averaging. In the illustrated FIG. 9 pattern 94,four A- and B-intervals are measured per sample. In the decoder, thesefour measurements are averaged together to produce the position signalfor the sample. Including more stripes and therefore more transitions inthe pattern will lower the noise by increasing the averaging, but willalso require a longer pattern, which lowers the sample rate. The scalingfactor that converts transition interval time to position signal isgiven by the slope of the stripes.

As the stripe transitions are sloped more azimuthally away from beingperpendicular to the servo track centerline, the timing betweentransitions will vary more with servo head position. These larger timingdifferences lower the noise in the position signal. Increased slopes,however, also make the servo pattern longer, lowering the sample rate.It should be noted that increased slopes decrease the signal strengthfrom the servo read head because of azimuth loss, which affects thenoise in the measurement of the transition times. All of these factorsshould be considered when determining the optimal servo pattern for agiven application. The pattern 94 illustrated in FIG. 9 represents apreferred design, however, different design objectives can be readilyaddressed by those skilled in the art by adjusting the pattern layoutand dimensions.

FIGS. 10, 11, and 12 show block diagrams of the signal decoder 36illustrated in FIG. 2. As described further below, the decoderpreferably includes error detection and correction circuitry. Thoseskilled in the art will appreciate that these two functions can beprovided in the same circuit or can be provided by separate circuitmodules. FIG. 10 shows that the decoder 36 receives the analog servoread head signal, such as illustrated in FIG. 7, from the servo readhead via the line 34 and converts the signal into pulsed logic signalsusing a peak detector 102. In the preferred embodiment, the outputsignal from the peak detector goes high on a positive-going transition(leading edge) and goes low on a negative-going transition, permittingthe decoder to distinguish between the two polarities.

As noted above, the position signal is decoded by a digital signaldecoder 36 (FIG. 2). The function of the decoder is to measure the A andB time intervals and perform the necessary calculations to make theposition signal available to the remainder of the servo control system.Additionally, error detection and correction may be applied within thedecoder. Those skilled in the art will recognize that while the decoderdesign and cooperation must be tailored for the particular servo trackpattern used, there are many ways to accomplish the function throughvarious hardware and software approaches. For illustrative purposes, adecoder and error correction circuit is illustrated in FIGS. 10 and 11,for use with simple patterns of the type shown in FIG. 4.

FIG. 7 shows the analog signal derived from a servo read head as aresult of reading a pattern as shown in FIG. 4. As shown in FIG. 10,this analog signal is converted to a digital signal by a peak detector102. The output of the peak detector switches from logic "low" to logic"high" upon detection of positive peaks, and from "high" to "low" atnegative peaks, which correspond to a single polarity of magnetictransition, as discussed previously.

In the decoder, a number of counters serve as timers for synchronizationand interval timing purposes. A start counter 104 detects start gaps 70(see FIG. 4) by looking for transition-free intervals longer than themaximum allowed within a burst. When a start gap is detected,synchronization and control circuits 111 are reset to begin decoding anew burst. As each peak in the servo pattern is encountered, theappropriate counters are enabled and reset to time the appropriate A andB intervals. A single "X" counter 106 times each A interval. Becauseconsecutive B intervals are contiguous, and a finite time is required tooutput a counter total and reset a counter, two "Y" counters Y1 108 andY2 110 alternate in timing the B intervals. The desired position signalis the ratio of A and B, which is calculated in this example circuit asfollows: Because full digital division requires extensive circuitry, itis advantageous to use a multiplier in combination with a ROM look-uptable when the expected range of B values is small (assuming tape speedvaries over a limited range). The B value (output of one of the two Ycounters) is selected by a Y1/Y2 counter selector 112 and is convertedto a 1/B value by a ROM table 116, whose output is multiplied by A in amultiplier 114. Thus, the raw position signal 118 comprises the valueA/B at the completion of each pair of A and B values (eight times ineach burst).

FIG. 11 shows a block diagram of a practical error detection andcorrection circuit to accompany the decoder shown in FIG. 10. Theillustrated circuitry performs error checking on each burst, and outputsa single position signal value for each burst. In the event no error isfound, the burst output is the average value of the eight individual A/Bvalues found within the burst. If an error is detected, a simple schemeis used to replace the current false burst output value with the mostrecent error-free value. These functions are accomplished as follows: Atransition counter 120 counts the number of transitions occurring ineach burst. Experimentally it has been determined that most errorsinvolve accidental detection of an extra transition, or failure todetect a legitimate transition, due to noise, dropouts, debris, or othercauses. When such errors occur, the transition counter 120 will count anumber of transitions per burst other than the correct number (eighteenin this example), and will output an error signal. Additional errordetection is accomplished by comparing the successive values of theeight A/B values generated within teach burst. A deviation accumulator124 sums four of the eight A/B values, and subtracts the remaining four,giving a deviation result that indicates the degree of inequality of theeight values. If this deviation value exceeds certain presentboundaries, a deviation limits detector 126 produces an error signal.These error signal are processed by an error gate/control logic 121.When no error is detected, the logic produces a burst data ready signalon a line 122 that pulses to indicate the availability of good burstdata from a burst average accumulator 128. If an error is detected, thenew burst average data is rejected, and replaced with the most recenterror-free value. This is accomplished with a latch 130 that causes thelast good burst average value from the accumulator 128 to be provided toa data selector 132. A burst error line 123 indicates whether thecurrent output value is a new error-free value or a previous held value.

The servo control system makes use of the burst error line 123 and theburst data ready line 122 to determine whether the position signalintegrity is sufficient for adequate servo control operation. Forexample, the system may reject position data after a certain number ofconsecutive errors are detected, or after a predetermined time intervalis exceeded without new error-free data. When such error conditionsoccur, the system may choose to accept position signal data from anotherredundant servo track or, if not servo tracks are producing error-freedata, the system may prevent data writing, so as to avoid possiblywriting new data off-track and accidentally erasing desired data onadjacent tracks. Such an arrangement is illustrated in FIG. 12.

The circuitry illustrated in FIG. 11 can accomplish error correctionmerely by substituting the most recent error-free value whenever anerror is detected. Those skilled in the art will recognize that otheralgorithms, such as substitution of an estimated current value, mayoffer certain advantages for the servo control system.

FIG. 12 shows a block diagram of a discriminator circuit 140 thatdetermines whether the burst data signal should be considered valid orinvalid. The burst data ready signal is received over the line 122 by atime-out timer 142 and a consecutive error counter 144. Burst errordeterminations are received from an error gate/control such as thatillustrated in FIG. 11. If the time-out timer 142 does not receive anerror-free servo burst signal during a predetermined time interval, thenthe time-out timer provides an error signal to an error gate 146. If theconsecutive error counter 144 counts a predetermined number ofconsecutive bursts having an error, then it provides an error indicationto the error gate. If neigh time-out timer nor the consecutive errorcounter indicate an error to the error gate, then the decoded signal isconsidered valid. A latch 148 then sets a data valid signal 150 to ahigh level, indicating a valid output.

The decoder described in connection with FIGS. 10-12 is a relativelysimple case that illustrates the principles of timing-based servopattern decoding and error detection and correction. The preferredembodiment of the invention uses the interleaved pattern shown in FIG.9, which has been optimized for a combination of wide servo track width,high signal-to-noise ration, high sampling rate, and good errordetection capability.

The preferred embodiment of a signal decoder constructed in accordancewith the present invention is illustrated in block diagram form in FIGS.13-16. FIG. 17 shows the path of a servo read head over a portion of theservo pattern from FIG. 9 with the resulting analog signal received bythe servo read head and the A and B intervals to be timed. The patternconsists of alternating bursts of four and five interleaved diamonds,separated by start gaps that exceed, at their narrowest points, thelength of any gaps encountered within bursts. This combination ofalternating groups of four and five stripes separated by recognizablegaps provides periodic synchronization information for the decoder.Because the intervals to be timed are interleaved and, in the case of Bintervals, are contiguous, the decoder is divided into two subdecodersindicated by a suffix of "1" or "2" that alternate in producing positionsignal information, each outputting every other position signal value.Each of these subdecoders times four A and four B intervals, which areshown in FIG. 17. The timing points labelled CLR1, CLR2, OUT1, and OUT2in FIG. 17 indicate the time points when each subdecoder is cleared andwhen each produces a position signal value. The main circuitry of thesubdecoders, including interval timing circuitry and error detectioncircuitry, is shown in FIGS. 13-16. The circuitry shown includes errordetection, but does not include error correction; this is assumed to behandled by the servo controller, using principles similar to thosediscussed above. Likewise, the quotient A/B is not calculated in thisdecoder, the servo controller performs this function. The details ofsuch circuitry can be readily determined by those skilled in the art inconjunction with this description.

While the interleaved A and B intervals could be timed with individualdedicated counters, the same function may be performed by a singleaccumulator in each subdecoder. For example, the timing of A values inthe first subdecoder is accomplished as follows: An X1 accumulator isinitially cleared (by CLR1) to a zero value. A transition counter TC1keeps track of the head location within the servo pattern (determined byhow many stripes have been crossed). When the location in the pattern isoutside of the A intervals, an X1 increment ROM produces a value of zeroto the X1 accumulator, holding its value at zero. At other points intime, the transition counter TC1 and increment ROM X1 provide to the X1accumulator an increment value equal to the number of A intervalscurrently being timed. The X1 accumulator adds this number to its totalon each clock cycle. In this way, the X1 accumulator serves the role ofmultiple parallel timers.

It should be noted that the X1 accumulator contains the sum of four Avalues after the intervals are complete, this is the desired A outputvalue for the burst group. In a similar manner as described above, Y1accumulator sums the four B intervals. A deviation accumulator D1alternately adds and subtracts both the A and B intervals in a mannersuch that its sum is zero if all A intervals have equal length and all Bintervals have equal length. The extent to which these equalities do nothold causes the sum in the D1 accumulator to deviate from zero.Comparators labelled DEV MAX and DEV MIN determine whether the deviationhas exceeded predetermined boundaries, which indicate an errorcondition. Error checking also includes transition counting, which isaccomplished by two magnitude comparators 1 and 2. Because the totalnumber of transitions expected is different (either 13 or 14) for eachsubdecoder, separate counters and magnitude checkers are provided foreach. Selection of which subdecoder is currently in use is determined bythe SELECT signal. This signal, as well as others depicted in FIG. 13,will be described in conjunction with FIGS. 14-16.

The DATA GOOD signal shown in FIG. 13 indicates whether an errorcondition has been detected either by the transition counters TC1, TC2or the deviation limit circuits DEV MAX and DEV MIN for the valuescurrently being produced on XOUT and YOUT data lines at the time of aDATA READY pulse (FIG. 16). The state of the DATA GOOD line is used bythe servo controller for error correction purposes.

FIG. 14 illustrates the generation of the PK signal as well as threeother intermediate signals, namely, a GAP, FOUR, and FIVE signal. The PKsignal is generated by a conventional peak detector 160 that typicallyis used in magnetic disk or tape drives to convert an analog signal intodigital pulses. The peak detector 160 differs slightly from those usedin most conventional drives in that is produces a pulse only onpositive-going and negative-going peaks. The peak detector 160 used inthe preferred embodiment should be well-known to those skilled in theart and needs no further explanation.

The output of the peak detector comprises the PK signal, which isprovided to the circuitry illustrated in FIG. 13 and also is provided toa down counter 162. The down counter also receives a clock signal 163from a system clock and receives a GAP length signal, which can be setby a user to a predetermined value, for example, corresponding to theseparation between stripes in a diamond of FIG. 9. A GAP signalcomprises a pulse generated by the down counter whenever a time intervalis detected beyond a predetermined gap length without a pulse from thepeak detector 160. That is, the down counter times out, or counts downto zero, if no PK pulse is detected after the gap length amount of time.For a given tape speed and servo pattern size, a suitable gap time limitis chosen, in the preferred embodiment, the tape speed is approximately2.0 meters per second and the pattern comprises groups of four and fivestripes 5 μm apart, and the preferred time limit is selected to be 3.75msec. As a result, the generated GAP signal comprises a pulse at eachgap between the groups of four and five servo pattern stripes. As notedabove, the spacing intervals 99 (FIG. 9) can be readily distinguishedfrom the internal pattern gaps 98 based on keeping track of the numberof diamond stripes encountered. This is described further below.

The PK signal and the GAP signal are used to produce the FOUR and FIVEsignals, respectively. The FOUR signal goes high whenever four servopattern stripes are detected after a pattern gap. The FIVE signal goeshigh whenever five servo pattern stripes are detected after a gap. An upcounter 164 receives the PK signal at a clock input and receives the GAPsignal at a clear input. The up counter provides its counting output tothe input lines of a 3-to-8line decoder 166. In a manner well-known tothose skilled in the art, the 3-to-8line decoder produces the FOUR andFIVE pulse signals.

FIG. 15 shows how the FOUR, FIVE, and GAP signals are used to producethe main control signals OUT1, OUT2, CLR1, and CLR2. For the patternillustrated in FIG. 9, the OUT1 and OUT2 signals are generated at everyspacing interval 99, whereas the CLR1 and CLR2 signals are generated atthe internal gaps 98. An array of flip-flops are used in conjunctionwith two OR gates to produce the control signal. The GAP signal isprovided to the clock input of the four flip-flops 172, 176, 178, 188.The inverse FIVE signal is provided to the first flip-flop 172 and to anOR gate 174. The OUT1 signal produces a single pulse whenever a gap isdetected after two FIVE pulses. Thus, the Q output of the firstflip-flop 172 is provided to the other input of the OR gate 174, whoseoutput is provided to the D-input line of a third flip-flop 178, and theinverse Q output of the first flip-flop 172 is provided as the clockinput to a fourth flip-flop 180. The D-input of the fourth flip-flop isgrounded. A fifth flip-flop 182 receives the Q output signal from thethird flip-flop 178 and also receives a system clock signal at its clockinput. The fifth flip-flop produces the OUT1 signal from its inverse Qoutput line.

The CLR2 line produces a single pulse whenever a GAP signal is detectedafter exactly one FIVE signal pulse. Thus, the Q output from the fourthflip-flop 180 is received at the D-input line of a sixth flip-flop 184,which also receives the system clock signal at its clock input. Theinverse Q output of the sixth flip-flop provides the CLR2 signal.

The OUT2 signal line produces a signal pulse when a GAP signal occursafter two FOUR signals, while the CLR1 line produces a signal pulse whena GAP signal occurs after exactly one FOUR signal. As illustrated inFIG. 15, this can be provided by connecting the inverted FOUR signal toone input of an OR gate 186 and also to the D-input of the secondflip-flop 176. The output Q of the second flip-flop 176 is provided tothe other input line of the OR gate 186. The output of the OR gate isprovided as the D-input to a seventh flip-flop 188. The GAP signal isprovided as the clock input to the seventh flip-flop. The Q output ofthe seventh flip-flop 188 is provided as the D-input to a ninthflip-flop 190. The ninth flip-flop receives the system clock signal atits clock input line. The inverted Q output from the ninth flip-flop 190produces the OUT2 signal.

The CLR1 signal is produced by a ninth flip-flop 192 whose D-input isgrounded and whose clock input is received from the inverted Q input ofthe second flip-flop 176. The Q output of the ninth flip-flop isprovided to the D-input of a tenth flip-flop 194. The tenth flip-flopreceives the system clock signal at its clock input line. The inverted Qinput of the tenth flip-flop 194 comprises the CLR1 signal.

FIG. 16 illustrates how a SELECT signal is generated and how adata-ready (DR) signal is generated. Whenever an OUT1 or OUT2 signalpulse occurs, data is ready to be output. That is, the servo head is atthe end of one diamond pattern, either a group of four interleaveddiamonds or a group of five interleaved diamonds. The SELECT signal isused to select the proper registers and data ready pulses. The SELECTsignal is produced from a J-K flip-flop 196 with its J input lineconnected to the OUT2 signal and its K input line connected to the OUT1signal. The clock input of the J-K flip-flop is connected to the systemclock signal. The Q output of the J-K flip-flop 196 produces the SELECTsignal. The OUT1 and OUT2 signals are connected to the input lines or anOR gate 198, whose output produces the data-ready (DR) signal.

The generation of the position signal relative to the interleaveddiamond pattern illustrated in FIG. 9 will be better understood withreference to the following drawings: FIG. 13, the logic circuit thatillustrates the generation of the position signal signals: FIG. 17, therepresentation of the interleaved diamond pattern and output signal: andFIG. 18, a chart illustrating the generation of the output signals andclear signals. As indicated in FIG. 13, the position signal comprisesalternating values designated XOUT and YOUT. As noted above, FIG. 13shows that there are two completely redundant signal generation systems,identified by a "1" suffix and "2" suffix, which take turns producingthe OUT and YOUT values. Thus, one XOUT value is generated by the X1elements, followed by a YOUT values generated by the Y1 elements,followed by a next XOUT value from the X2 elements, a next YOUT valuefrom the Y2 elements, a next XOUT value from the X1 elements, and soforth. The sequent of values comprises the position signal. Thedescription of the circuit operation initially will refer only to thefirst of the redundant signal generation systems, indicated by a "1"suffix.

The position signal is the sum of four A-interval values divided by thesum of four B-interval values described above in connection with FIGS.4-6 and FIGS. 8 and 9. The accumulators X1, X2, Y1, and Y2 shown in FIG.13 can perform the division, or multiplication by reciprocals, of the Aand B values and then add the quotients, or can calculate the sums andthen perform the division operation, to generate the XOUT and YOUTvalues. The A intervals and B intervals are graphically represented inFIG. 17.

FIG. 17 shows that the A intervals A1, A2, A3, and A4 overlap oneanother in time, as do the corresponding B intervals B1, B2, B3, and B4.A separate counter could be used to time each A and B interval; however,this approach would require eight counters. As noted above, in thepreferred embodiment, the summation is instead achieved using twoparallel signal generation systems having paired accumulators. Asillustrated in FIG. 13, the first signal generation system includes twopaired accumulators X1 and Y1, while the second signal generation systemincludes two accumulators X2 and Y2. Each accumulator has input linesfor "clear" and "increment", and it should be understood that each alsoreceives a clock input (not illustrated). With each clock cycle, theaccumulator adds the amount of the increment obtained from an incrementROM (labeled "INC ROM") to the data output signal. Each accumulator addsan increment of zero, one, two, three, or four, depending on the servopattern band just crossed by the servo head. A pulse on the respectivesignal generation system clear line (CLR1 or CLR2) resets theaccumulator output to zero. The accumulators get their incrementinstruction, the amount to add with each clock cycle, from the incrementROMs. The increment ROMs, in turn, are addressed by transition countersTC1 and TC2.

In operation, when a CLR1 pulse occurs, the first transition counter TC1resets to zero and its associated accumulators X1, Y1, and D1 also arereset to zero. As the servo head moves along the servo pattern after theCLR1 pulse, as illustrated in FIG. 17, it crosses a group of four servopattern stripes, then crosses two groups of five stripes. The transitioncounters count peak (PK) pulses to keep track of how many servo patternstripes have been crossed. On the first PK pulse after the CLR1 signalis received, the system begins timing the first B interval. On thesecond PK pulse, the second B interval timing begins, and so on. On thesixth PK pulse after the CLR1 signal, the first A interval timingbegins. On the seventh PK pulse, the second A interval timing begins.This continues such that, on the eleventh PK pulse after the CLR1signal, representing the eleventh servo pattern stripe crossed, thefirst A interval and the first B interval timing end. On the fourteenthPK pulse after the CLR1, all A and B intervals have ended and the sumsare ready to be output. The fourteenth servo pattern stripe, after apair of five-stripe groups, is when the OUT1 pulse occurs, producing anoutput value (FIG. 15).

The accumulators, using the increment data in the increment ROMs,automatically add up the intervals as needed. FIG. 18 shows theincrement data that is stored in the respective increment ROMs. In FIG.18, the ADDRESS column is the transition counter output value thatindicates when servo pattern stripe after the associated CLR1 or CLR2signal has just been crossed. The columns X1, X2, . . . , D2 show whatincrement values will be added to each respective accumulator of FIG. 13for each clock cycle. It should be noted that the address refers to thenumber of PK pulses following the corresponding clear signal. Thus, theincrement values in the X1 column are indexed according to the number ofPK pulses received after a CLR1 signal, while the increment values inthe X2 column are indexed according to PK pulses after a CLR2 signal.

The operation of the X1 accumulator next will be described in greaterdetail. The other accumulators operate in a similar manner. From FIG.17, it should be clear that the sixth servo pattern stripe crossed aftera CLR1 signal starts the timing of the first A interval for afive-stripe diamond. This can be seen from examination of the servopattern stripes, the head output analog signal, and the second group ofA intervals. Thus, the transition counter TC1 output value that isproduced from counting PK pulses is equal to six and the correspondingincrement ROM address is equal to six. From FIG. 18, the amount ofincrement to the X1 accumulator is one.

On the seventh PK pulse following a CLR1 signal, the first A intervalcontinues to be timed while the second A interval timing begins.Therefore, after the seventh PK pulse indicating the crossing of theseventh servo pattern stripe, the ROM address is seven and from FIG. 18it is clear that the accumulator X1 increments two on each clock cycle.Similarly, after the eighth servo pattern stripe is crossed, three Aintervals A1, A2, and A3 are simultaneously timed, so the accumulator isincremented by three on each clock cycle. On the ninth servo patternstripe, the accumulator is incremented by four. On the eleventh servopattern stripe, the first A interval A1 has ended, and therefore onlythree intervals continue to be timed. Therefore, the increment to the X1accumulator is reduced to three, as indicated in the FIG. 18 table entryfor ROM address eleven. After the fourteenth servo pattern stripe, all Aintervals are completed and therefore the accumulator increment ischanged to zero. That is, the accumulator already contains the sum ofthe four A intervals and the output value is ready to be produced, afteran OUT1 pulse occurs. Similarly, the Y1 accumulator will have beentiming the B intervals and also has its data ready to be output.

The set of accumulators X2 and Y2 of the second signal generation systemoperate in the same way, starting from the CLR2 signal and completing intime for the OUT2 pulse (FIG. 15). Thus, the sixth servo pattern stripecrossed after a CLR2 signal corresponds to the first servo patternstripe of a four-diamond group. Therefore, the A1 interval for thesecond signal generation group begins and the X2 accumulator should beincremented by one. This is shown by the corresponding value in the FIG.18 table for ROM address six of the X2 column. At a tape speed ofapproximately 2.0 meters per second, the combination of the two sets ofaccumulators provides new position signal data at a rate ofapproximately 18 kHz.

FIG. 13 shows that the outputs of the accumulators are routed throughrespective selectors X-SELECT, Y-SELECT, and D-SELECT that choose whichof the two signal generation systems has the current output value thatshould be output. The selection is governed by the SELECT data signal,described above in conjunction with FIG. 16. After an OUT1 pulse, theset of accumulators from the second signal generation system becomesactive, and after an OUT2 pulse, the set of accumulators from the firstsignal generation system becomes active. Thus, for the interleaveddiamond servo pattern illustrated in FIG. 17, the first set ofaccumulators X1, Y1, D1 becomes active after an OUT2 pulse, which occursafter two four-stripe groups, while the second set of accumulators X2,Y2, D2 becomes active after an OUT1 pulse, which occurs after twofive-stripe groups.

In the preferred embodiment illustrated in FIG. 14, error checking isperformed to detect missing or extra transitions and to detect servopattern stripes that are erroneously read in a slightly shiftedposition. Details of error correction that can be performed after errordetection are not illustrated in FIG. 14 but those skilled in the artwill readily be able to construct such circuitry, in view of thediscussion above concerning FIGS. 10-12. In FIG. 13, missing or extradetected stripes are detected by the transition counters TC1 and TC2that count every PK pulse. When an output signal pulse OUT1 or OUT2occurs, magnitude comparators check to see if the correct number oftransitions (either 13 or 14, as indicated) have been detected. Forexample, in the case of the first set of accumulators, the predeterminednumber of transitions is fourteen whereas for the second set ofaccumulators, the predetermined number is thirteen. If a number otherthan the predetermined number is detected, then the data-good (DG)signal produced by the SELECT block will be false. The system decoder 36(FIG. 2) detects the DG signal and thereby is warned that the data isbad and takes predetermined corrective action. In the preferredembodiment, for example, the corrective action comprises maintaining theoutput signal at is previous value.

If the PK pulse from one servo pattern stripe is accidentally shifted intime, then all A interval values and B interval values will not have thesame value. The system illustrated in FIG. 13 provides deviationaccumulators D1 and D2 that add and subtract the individual A and Bintervals in a way that should provide a zero result. If any servopattern stripe is shifted in time, the result will be non-zero, eitherpositive or negative. A maximum comparator DEV_(max) and a minimumcomparator DEV_(min) check the D1 and D2 deviation accumulator output topermit servo pattern stripe shifts to be larger than a predeterminedminimum amount, which permits normal noise within the system to causeacceptably small errors, but does not permit shifts to be larger than apredetermined maximum amount, which indicates an error. If thedifference is less than the maximum value and less than or equal to theminimum value, then the output of the corresponding deviationaccumulator D1 or D2 goes high and, along with output from the SELECTdata signal, produces the data-good (DG) signal. In this way, thedeviation check circuitry also prevents random noise from beingconsidered valid output in the event that random noise produces suitablepatterns to produce an OUT1 or an OUT2 signal pulse.

The signal decoder and position signal circuitry described above useservo control information that comprises a repeating servo pattern ofstripes having magnetic flux transitions that extend continuously acrossthe width of servo information tracks in the translating direction in anazimuthal slope. The signal decoder receives the analog servo read headsignal and generates a position signal that is a function of the ratioof two intervals derived from the servo pattern. This provides a servocontrol system that is independent of tape speed and therefore isinsensitive to speed variations. Those skilled in the art willappreciate that a variety of techniques can be used to produce the servopatterns illustrated in FIGS. 4-9 in magnetic storage media, such asmagnetic tape. A variety of systems for producing the servo patternsused by the decoder will be described next.

FIG. 19 shows a magnetic drum system 300 for producing the servopatterns described above. Magnetic tape 302 onto which the servopatterns are to be recorded is wound around a curved portion of thecircumference 304 of a drum 306 such that the curved portion is adjacentan electromagnet 308 on the opposite side of the tape that projects amagnetic field of flux outwardly toward the tape. A sequence of raisedbands is deposited onto the circumference of the drum in the desiredservo pattern. For example, the bands deposited onto the circumferentialportion 304 of the drum illustrated in FIG. 20 produce a servo patternon the tape 302 that is the same as that illustrated in FIG. 4. Otherdetails of a drum system implementation for producing the servo patternsare well-known to those skilled in the art and do not form part of theinvention described herein. See, for example, U.S. Pat. No. 3,869,711 toBernard.

Those skilled in the art will appreciate that the drum portion 304shields those lengths of the magnetic tape with which the bands havecontact while the external electromagnet 308 projects a magnetic fieldonto the tape, leaving the desired servo pattern flux transition bands.The drum pattern bands 310 preferably are deposited usingphotolithographic techniques, as such techniques provide the extremeaccuracy needed for accurate reproduction of the servo patterns.Preferably, the bands are constructed of a nickel iron or permalloymaterial on a nonmagnetic drum.

A preferred method for producing the patterns is with a multiple gapservo write head. The multiple gap heads of the preferred embodiment areproduced from photographic techniques known to those skilled in the art.FIG. 21 shows a multiple gap servo write head 400 constructed inaccordance with the present invention. The head illustrated in FIG. 21comprises a ferrite ring 402 with a patterned NiFe pole piece region404. Two ferrite blocks 406, 408 form the bulk of the magnetic head andare separated by a glass spacer 411.

In constructing the head, the ferrite blocks 406, 408 and glass spacer411 are first bonded together with an epoxy glue or with glass bondingtechniques. The resulting structure is then lapped to produce thedesired front contour, which comprises the tape bearing surface. In thepreferred embodiment, a cylindrical front contour surface is provided.Cross-slots 412 are cut into the head to remove included air when thehead is in operation with magnetic tape.

As shown in FIG. 22, a conducting seedlayer 416 is then deposited on thefront contour surface. In the preferred embodiment, 800 angstroms ofNiFe have been used. Photoresist material is then deposited on the frontsurface and patterned in the shape of the desired servo patterns 414.The patterning of the cylindrical surface can be done by eithercontact-exposure or projection-exposure techniques familiar to thoseskilled in the art. Because high resolution is only required for theservo patterns located at the apex of the cylindrical contour, standardplanar exposure techniques can be used. In the preferred embodiment, thephotoresist lines which define the gap regions are 2 μm wide and 3.5 μmtall.

After the desired gap structures are formed in the photoresist, Ni₂₅Fe₅₅ material 418 is plated to a thickness of approximately 2 μm on theseedlayer 416 wherever the photoresist has been removed. The remainingphotoresist material is then removed. A wear-resistant overcoat 420 isthen deposited over the front contoured surface to protect it. In thepreferred embodiment, this overcoat is a laminated NiFeN/FeN structurewith a total thickness of approximately 3000 angstroms. Alternativeovercoat materials that can be used are, for example, diamond-likecarbon or other wear-resistant materials.

Finally, as shown in FIG. 21, a coil 420 is wound around one of theferrite blocks 408 through a wiring slot 422 to complete the head. Theflux through each gap is in the same plane as the lithography. Thislimits the gap width to the resolution of lithographic techniques, butallows arbitrarily complex gap shapes within that restriction. Thus, thestraight diagonal gaps needed for the servo patterns illustrated inFIGS. 4-9 are easily fabricated in this horizontal head design describedabove. Those skilled in the art will appreciate that a much more complexprocess would be needed to produce the desired gap structure in avertical head because of the limits of planar processing.

One novel aspect of the head 400 is its use of magnetic saturationphenomena to simplify its design. The writing gaps 414, shown in greaterdetail in FIG. 23, are contained within a continuous sheet of magneticNiFe. Conventional wisdom would dictate that the magnetic field in thesegaps should be very small when this head is energized because almost allof the flux would flow through the low-reluctance NiFe rather thanthrough the high-reluctance writing gaps. The gaps appear to be shuntedby the sheet of NiFe. However, at larger currents the shunting regionsof NiFe become magnetically saturated, causing the permeability to dropsharply. As saturation becomes more severe, the writing gaps become thepreferred path for the additional flux. At high writing currents, thisdesign generates the necessary gap fields to adequately write magnetictape. This design provides an almost completely smooth surface for thetape to run over. More conventional designs would require wide isolationgaps to channel the magnetic flux into the writing gaps. Such isolationgaps provide high-pressure edges which will be subject to wear by thetape. These wide gaps also provide regions for tape debris to accumulatewhich can cause unwanted spacing between the head and tape. It should benoted that extra flares 432 are added to the writing gaps 430 to sharplydemarcate the written pattern. If the flares are not present, the fieldacross the writing gaps decreases at the ends. The flares act tomaintain nearly the full writing field up to the end of the writinggaps.

Saturation effects are also used effectively to eliminate negativeeffects due to the magnetic seedlayer and the preferred magnetic wearovercoat. These layers are magnetic and cover the entire front surfaceof the head, including across the writing gaps 414. This shorting of thegaps would cause a problem except that these films are saturated at verylow currents and cause no effect at higher writing currents. Thoseskilled in the art will recognize that the advantageous use of thesesaturation effects simplifies the design and improves the performance ofthis head.

The preferred embodiment of the servo write head uses a cylindricallycontoured head with cross-slots to maintain good contact between thehead and the tape. Other techniques for maintaining this contact canalso be used. In particular, a flat head with a small-radius edge can beused by overwrapping the tape around the edge. FIG. 24 illustrates thistechnique. The head 900 has a planar front surface 902. The tape 904contacts the head with a slight overwrap (for example, 1°). The actionof passing the tape over the overwrapped corner of the head acts toremove the layer of air between the head and the tape. The tape liftsslightly off the head near the corner because of its finite modulus ofelasticity, but then comes into contact with the head. This techniquemay be used to maintain head-tape contact. Those skilled in the art willrecognize that certain simplifications in the head fabrication processmay be used to advantage by eliminating the cylindrical contour andcross-slots from the head design.

FIG. 25 illustrates the process of producing a magnetic tape having theservo patterns illustrated above using a pattern recording system 502.The system 502 can be provided, for example, in the tape drive 12illustrated in FIG. 1. In particular, FIG. 25 illustrates the process ofproducing a magnetic tape having the servo pattern of FIG. 9 and shows amagnetic tape 504 in a top view 506 and in a side view 508 as it ispassed in contact with a write head 510 such as that illustrated inFIGS. 21 and 23. The tape is passed in the direction indicated by thearrow 512.

The tape write head 510 ordinarily is not energized, but is periodicallyenergized with a current pulse of predetermined polarity atpredetermined times. That is, the head is switched between a zerocurrent and a current of a single polarity. Those skilled in the artwill note that this differs from conventional schemes, in which amagnetic write head is switched back and forth between opposite polaritycurrents. To produce the desired servo pattern on the tape 504, the tapeis moved at a predetermined velocity while the write head 510 isintermittently pulsed with current. The intermittent current pulses orthe write head produce flux patterns on the tape that tare a copy of thehead gap structure, as illustrated by the representation of the tapepattern 514 in FIG. 25. It should be clear from FIG. 25 that the twochevron-shaped write gaps are spaced apart sufficiently such that twoopposed stripe bands, or diamonds, are recorded with each current pulsethrough the write head 510 and that the current pulses are timed tocreate the interleaved diamond pattern illustrated in FIG. 9, where agroup of four interleaved diamonds is followed by a group of fiveinterleaved diamonds.

FIG. 25 illustrates that the space 511 between the head write gaps isselected so that the interleaved pattern can be written in a single passof the tape. The magnetic storage medium is moved in a head transducingdirection at a predetermined velocity and the servo write head isenergized with a predetermined polarity pulse to generate magnetic fluxand automatically record one servo pattern transition stripe of thefirst azimuthal orientation and one servo pattern stripe of the secondazimuthal orientation in a track on the tape with each energization. Thehead is repeatedly energized until the stripes recorded in the tapecomprise one of the interleaved diamond groups. More particularly, thespacing between the servo write gaps is selected so that, at the tapewriting speed, the transition stripe recorded by the trailing gap 513 atthe last current pulse of a four-stripe or five-stripe group liescompletely between the first stripes recorded by the leading gap 515 andtrailing gap at the first current pulse of the group. Thus, after fouror five activations of the head, as appropriate, the desired interleavedservo pattern is obtained.

Similarly, the synchronization feature spacing intervals are formed bycontinuing to move the tape at the predetermined velocity withoutenergizing the servo write head to produce a servo pattern stripe. Theextent of the spacing interval in the transducing direction isdetermined by the length of time the head is not energized and by thepredetermined tape speed. Preferably, the time without energizing thewrite head is sufficiently long such that all of the stripes written bya group of pulses lie completely beyond the stripes written by theprevious groups of pulses. That is, all transition stripes of one grouphave passed by the trailing gap 513 before any stripes of the next groupare written to the tape.

To write a non-interleaved pattern, such as illustrated in FIG. 8, thewrite gap spacing and pulsing of the head is such that the transitionstrip recorded by the trailing gap during each current pulse liescompletely beyond the stripe recorded by the leading gap during theprevious current pulse. That is, the stripe written by the leading gapis moved past the trailing gap before the next energization of the writehead. The synchronization features between stripe groups are formed bydelaying energization of the servo write head to produce a servo patternstripe for a sufficiently long time so that the minimum spacing alongthe transducing direction between the last stripe recorded by theleading gap during the last current pulse of a group and the first striprecorded by the trailing gap during the first current pulse of thesubsequent group is greater than the maximum distance in the transducingdirection between any pair of sequential stripes within a group.

As illustrated in FIG. 25, a programmable pattern generator 516 of theservo pattern recording system 502 generates pulses that are provided toa pulse generator 518 that causes the intermittent energizing of thewrite head 510. Because the pulse width is finite and the tape is movingat a predetermined velocity, the servo flux patterns recorded onto thetape 504 are elongated versions of the actual gaps of the write head.The flux patterns recorded on the tape are wider than the gaps on thewrite head by the product of the tape velocity and the pulse width.

The servo pattern recording system 502 can operate with either AC or DCerased magnetic tape. If the magnetic tape 504 is AC-erased, meaningthat the tape has zero magnetization, then the tape is magnetized withone polarity over the gap regions when the write head 510 is energized.The remainder of the tape is left with zero magnetization. If themagnetic tape is DC-erased, meaning that the tape is magnetized in onepolarity, then the current through the write head 510 must be directedsuch that the recorded flux pattern stripes are magnetized in theopposite polarity. The resulting recorded pattern then consists oftransitions between magnetized regions of opposite polarity. The signalproduced when a servo pattern is read back from a DC-erased tape willhave approximately twice the amplitude of the signal produced from anAC-erased tape. In the preferred embodiment, however, an AC-erased tapeis used to prevent producing a signal so large that the servo read headbecomes saturated. The magnitude of the write current can also bereduced to decrease the magnetization of the written regions of thetape, lowering the readback signal.

The pattern generator 516 in FIG. 25 may be constructed with severaltechniques which are familiar to one skilled in the art. For example,the required pulse pattern could be recorded in a programmable read-onlymemory (PROM) and cycled through with an appropriate addressing circuit.Alternatively, the required pulse pattern could be produced by acollection of suitable counters and associated logic. These techniquesare familiar to those skilled in the art and require no furtherexplanation.

It also should be appreciated that the accuracy of the servo patternrecorded on the tape depends on the accuracy of the pattern generationtiming and of the tape velocity. The pattern generation timingpreferably is crystal controlled and therefore is very accurate andstable. The tape velocity, however, is more difficult to control. In thepreferred embodiment, a tape velocity accuracy of 0.1% is required. Analternative to obtaining such accuracy is to measure the tape velocitynear the write head and adjust the timing of the pattern generator tocorrect for tape velocity errors. Measuring the tape velocity can beaccomplished, for example, with an accurate shaft encoder 505 rotated bythe tape or with a laser doppler device. The details of such a tapevelocity measurement system should be clear to those skilled in the art.

After the pattern pulses are generated, they must be converted intocurrent pulses through the write head. In the preferred embodiment,pulse generator circuitry produces pulses with a duration of 150 ns,with up to 3 amps of peak current and a rise and fall time of less than50 ns. Those skilled in the art will recognize that such a pulsegenerator can be constructed with, for example, a power MOSFET switchand a current-limiting resistor. These techniques will be readilyapparent to those skilled in the art without further explanation.

FIG. 26 is a schematic diagram of the tape writing system 502illustrating tape verification elements and showing that the tape 504 ispassed from a supply reel 520 to a take-up reel 522 as the servo patternis recorded onto the tape. The pattern generator 516 produces thepattern pulses, which are provided to the servo write head pulsegenerator 518 that intermittently energizes the write head 510. Afterthe tape 504 is recorded with the servo pattern, the pattern must beverified to assure high quality. A servo read head 524 reads thejust-recorded servo pattern and provides a servo signal to apre-amplifier 526. The pre-amplifier provides an amplified version ofthe servo signal to a pattern verifier 528 that performs a variety ofverifying operations, such as checking the servo pattern, signalamplitude, dropout rate, and consistency of redundant servo tracks. Theverifier causes a bad-tape marking head 530 to place a magnetic mark onthe tape 504 if any errors are found so that bad sections of tape arenot loaded into a tape cartridge (FIG. 1).

Although this discussion has focused on dedicated servo trackembodiments, this servo system is also applicable to embedded servoembodiments. In dedicated servo track systems, certain tracks on thetape are used exclusively for servo patterns. In operation a servo readelement is always over one of these servo tracks while other elementsare used for reading and writing data. Embedded servo systems spatiallyseparate servo patterns and data blocks on the same track. With thisapproach, a single element can be used for reading both servoinformation and data. The embedded servo approach decreases the servosample rate an the data rate because a single element is used for both.One disadvantage to using the same head element for servo and datareading is that using a narrow servo read head is, for all practicalpurposes, precluded. However, other advantages are obtained, such asdecreasing the number of elements needed in a head module and decreasingoffset errors which result from using separate servo and data elements.Those skilled in the art will appreciate that the servo system describedhere can be extended to apply to embedded servo applications.

Thus, described above is a servo pattern of repeating magnetic fluxtransitions that extend across the width of each servo track such thatthey produce a servo position information signal that variescontinuously as a servo read head is moved across the width of the servotrack in the translating direction and the tape is moved beneath thehead in the transducing direction, permitting the interval betweentransitions to be timed to thereby indicate the relative position of themagnetic head within the track. Also described above are a variety ofservo write heads suitable for generating the servo pattern, including apreferred method of constructing a multiple gap servo write head. Alsodescribed above is a servo pattern writing system, including a tapeverification system to ensure accurate reproduction of the servopatterns on tape.

The present invention has been described above in terms of presentlypreferred embodiments so that an understanding of the present inventioncan be conveyed. There are, however, many configurations for servodecoders, servo patterns, servo control systems, storage media, servowriting systems, data storage systems, and servo write heads notspecifically described herein, but with which the present invention isapplicable. The present invention should therefore not be seen aslimited to the particular embodiments described herein, but rather, itshould be understood that the present invention has wide applicabilitywith respect to servo decoders, servo patterns, and servo write headsgenerally. All modifications, variations, or equivalent arrangementsthat are within the scope of the attached claims should therefore beconsidered to be within the scope of the invention.

We claim:
 1. A servo control system for positioning a magnetic headadjacent a surface of a moving magnetic storage medium for reading aservo pattern recorded in at least one track on the storage mediumsurface, the system comprising:a head assembly having at least one servoread head for reading the servo pattern on the storage medium in atransducing direction and generating a read head signal representativeof the servo pattern; a servo decoder that receives the read head signaland decodes it to generate a position signal that indicates the positionof the read head relative to the servo pattern; a translation assemblythat is activated to position the head assembly relative to the storagemedium; a servo controller that activates the translation assembly inaccordance with the position signal, wherein:the servo decoder includingmeans for generating a substantially speed invariant position signalthat is based on a ratio of time intervals between a plurality ofpredetermined pairs of magnetic flux transitions in the servo pattern,and means for detecting errors in the substantially speed invariantposition signal by means of pattern recognition of the read head signal,such that the servo decoder correlates the read head signal withpredetermined patterns recorded on the storage medium and, if the signaldoes not correlate within an error limit, then the servo decoderindicates an error condition.
 2. A servo control system as defined inclaim 1, wherein the servo decoder means for detecting errors detects aservo pattern that comprises a cyclic sequence of magnetic fluxtransitions that include periodic synchronization features and the servodecoder correlates the servo head signal by counting the number oftransitions occurring between synchronization features and comparing thenumber with the number of transitions in the predetermined servopattern, such that if the number of transitions detected by the servodecoder does not equal the number known to exist in the predeterminedservo pattern, then the servo decoder indicates an error condition.
 3. Aservo control system as defined in claim 2, wherein the servo decodermeans for detecting errors decodes a read head signal produced from aservo pattern that comprises a cyclic sequence of magnetic fluxtransitions that extend continuously across the width of the track anddefine servo pattern stripes such that the servo read head signal variesas the magnetic head is moved across the width of the track, the stripesincluding at least a first azimuthal orientation and a second azimuthalorientation, such that the first azimuthal orientation is not parallelto the second azimuthal orientation and the stripes are arranged ingroups having a plurality of sequential stripes at the first azimuthalorientation followed by a plurality of sequential stripes at the secondazimuthal orientation.
 4. A servo control system as defined in claim 3,wherein the synchronization features of the servo pattern comprisetransition-free spaces that are free of transitions of at least onepolarity whose minimum length in the transducing direction exceeds themaximum length in the transducing direction between consecutivetransitions of that polarity within a group of stripes of the sameazimuthal orientation.
 5. A servo control system as defined in claim 2,wherein the servo decoder means for detecting errors decodes a read headsignal produced from a servo pattern that comprises a cyclic sequence ofmagnetic flux transitions that extend continuously across the width ofthe track and define servo pattern stripes such that the servo read headsignal varies as the magnetic head is moved across the width of thetrack, the stripes including at least a first azimuthal orientation anda second azimuthal orientation, such that the first azimuthalorientation is not parallel to the second azimuthal orientation and thestripes are arranged in groups containing a plurality of sequentialsubgroups, each subgroup containing stripes at more than one azimuthalorientation, the groups being separated by synchronization featuresdetectable by the servo decoder.
 6. A servo control system as defined inclaim 5, wherein the synchronization features of the servo patterncomprise transition-free spaces that are free of transitions of at leastone polarity whose minimum length in the transducing direction exceedsthe maximum length in the transducing direction between consecutivetransitions of that polarity within a group of stripes of the sameazimuthal orientation.
 7. A servo control system as defined in claim 1,wherein the servo decoder means for detecting errors detects a servopattern that comprises a cyclic sequence of flux transitions and theservo decoder correlates the read head signal by comparing the durationof the time intervals to a predetermined time interval relationship,such that if the relationship between the time intervals does not equalthe predetermined relationship within an error limit, then the decoderindicates an error condition.
 8. A servo control system as defined inclaim 7, wherein the servo decoder means for detecting errors comparestime intervals between pairs of transitions for equality, such that ifthe time intervals differ from one another by more than a predeterminederror limit, then the servo decoder indicates an error condition.
 9. Aservo control system as defined in claim 7, wherein the servo decodermeans for detecting errors compares sums of a plurality of timeintervals between pairs of transitions for equality, such that if thesums differ from one another by more than a predetermined error limit,then the servo decoder indicates an error condition.
 10. A servo controlsystem as defined in claim 1, wherein the substantially speed invariantposition signal comprises a sequence of values and, if the servo decoderindicates an error condition in the read head signal, the servo controlsystem replaces the current position signal value with a value derivedfrom one or more position signal values that occurred before the errorcondition.
 11. A servo control system as defined in claim 10, whereinthe replacement value is the last position signal value generated by theservo decoder before the error condition was indicated.
 12. A servocontrol system as defined in claim 1, wherein the substantially speedinvariant position signal comprises a sequence of values and, if theservo decoder indicates an error condition in the read head signal, theservo control system discards the current position signal value.
 13. Aservo control system as defined in claim 12, wherein the servo controlsystem includes one or more additional read heads for reading respectiveadditional servo patterns on the storage medium and generatingrespective additional read head signals representative of the additionalservo patterns and one or more respective additional servo decoders thatreceive the additional head signals and decode them to generaterespective additional position signals that indicate the position of theadditional read heads relative to the servo patterns, and the servocontrol system replaces the discarded current position signal value witha value derived from one or more of the additional servo position signalvalues that are not in an error condition.
 14. A data storage systemcomprising:a magnetic storage medium having a servo pattern recorded onat least one servo track; drive means for moving the magnetic storagemedium relative to a magnetic head assembly; a magnetic head assemblythat is moved sufficiently close to a surface of the moving magneticstorage medium for reading the servo pattern recorded on the storagemedium surface and for generating a servo read head signal, the magnetichead assembly including at least one data head for reading and writingdata and at least one servo read head for reading servo information ontracks of the storage medium; a servo control system for positioning themagnetic head assembly adjacent the surface of the moving magneticstorage medium for reading the servo pattern recorded in at least onetrack of the storage medium surface, a servo decoder that receives theservo read head signal and decodes it to generate a position signal thatindicates the position of the servo read head relative to the servopattern, a translation assembly that is activated to position the headassembly relative to the storage medium, and a servo controller thatactivates the translation assembly in accordance with the positionsignal, wherein the servo decoder includes means for generating asubstantially speed variant position signal that is based on a ratio oftime intervals between a plurality of predetermined pairs of magneticflux transitions in the servo pattern, and means for detecting errors inthe substantially speed invariant position signal by means of patternrecognition of the read head signal, such that the servo decodercorrelates the read head signal with predetermined patterns recorded onthe storage medium and, if the signal does not correlate within an errorlimit, then the servo decoder indicates an error condition.
 15. A datastorage system as defined in claim 14, wherein the servo decoder meansfor detecting errors detects a servo pattern that comprises a cyclicsequence of magnetic flux transitions that include periodicsynchronization features and the servo decoder correlates the read headsignal by counting the number of transitions occurring betweensynchronization features and comparing the number with the number oftransitions in the predetermined servo pattern, such that if the numberof transitions detected by the servo decoder does not equal the numberknown to exist in the predetermined servo pattern, then the servodecoder indicates an error condition.
 16. A data storage system asdefined in claim 15, wherein the servo decoder means for detectingerrors decodes a read head signal produced from a servo pattern thatcomprises a cyclic sequence of magnetic flux transitions that extendcontinuously across the width of the track and define servo patternstripes such that the servo read head signal varies as the magnetic headis moved across the width of the track, the stripes including at least afirst azimuthal orientation of a second azimuthal orientation, such thatthe first azimuthal orientation is not parallel to the second azimuthalorientation and the stripes are arranged in groups having a plurality ofsequential stripes at the first azimuthal orientation followed by aplurality of sequential stripes at the second azimuthal orientation. 17.A data storage system as defined in claim 16, wherein thesynchronization features of the servo pattern comprise transition-freespaces that are free of transitions of at least one polarity whoseminimum length in the transducing direction exceeds the maximum lengthin the transducing direction between consecutive transitions of thatpolarity within a group of stripes of the same azimuthal orientation.18. A data storage system as defined in claim 15, wherein the servodecoder means for detecting errors decodes a read head signal producedfrom a servo pattern that comprises a cyclic sequence of magnetic fluxtransitions that extend continuously across the width of the track anddefine servo pattern stripes such that the servo read head signal variesas the magnetic head is moved across the width of the track, the stripesincluding at least a first azimuthal orientation and a second azimuthalorientation, such that the first azimuthal orientation is not parallelto the second azimuthal orientation and the strips are arranged ingroups containing a plurality of sequential subgroups, each subgroupcontaining stripes at more than one azimuthal orientation, the groupsbeing separated by synchronization features detectable by the servodecoder.
 19. A data storage system as defined in claim 18, wherein thesynchronization features of the servo pattern comprise transition-freespaces that are free of transitions of at least one polarity whoseminimum length in the transducing direction exceeds the maximum lengthin the transducing direction between consecutive transitions of thatpolarity within a group of stripes of the same azimuthal orientation.20. A data storage system as defined in claim 14, wherein thesubstantially speed invariant position signal comprises a sequence ofvalues, and, if the servo decoder indicates an error condition in theread head signal, the servo control system replaces the current positionsignal value with a value derived from one or more position signalvalues that occurred before the error condition.
 21. A data storagesystem as defined in claim 20, wherein the replacement value is the lastposition signal value generated by the servo decoder before the errorcondition was indicated.
 22. A data storage system as defined in claim21, wherein the servo control system further includes one or moreadditional read heads for reading respective additional servo patternson the storage medium and generating respective additional read headsignals representative of the additional servo patterns and one or morerespective additional servo decoders that receive the additional headsignals and decode them to generate respective additional positionsignals that indicate the position of the additional read heads relativeto the servo patterns, and the servo control system replaces thediscarded current position signal value with a value derived from one ormore of the additional servo position signal values that are not in anerror condition.